SCIENTIFIC ABSTRACT HIV infection-associated B-cell hyperactivation plays a central role in the genesis of AIDS-associated non- Hodgkin lymphoma (AIDS-NHL), as it is associated with molecular processes that lead to the seminal molecular lesions for NHL. In our prior work, we observed evidence for HIV-associated B-cell activation, including elevated numbers of B cells overexpressing transferrin receptor 1 (TfR1), a marker of B-cell activation, for several years preceding NHL diagnosis. We have developed an antibody (ch128.1) and an antibody-avidin fusion protein (ch128.1Av), specific for human TfR1. ch128.1Av shows enhanced in vitro cytotoxicity against human malignant B cells overexpressing TfR1, including multiple myeloma (MM) and NHL cells, when compared to the parental antibody without avidin (ch128.1). Cytotoxicity is due to the ability of ch128.1Av, and to a lesser extent ch128.1, to decrease cell-surface TfR1 leading to lethal iron deprivation. ch128.1Av is also a universal delivery system and its cytotoxicity can be enhanced by conjugating it to biotinylated drugs such as the plant toxin saporin; making it a unique molecule capable of a two-pronged attack against malignant cells. However, ch128.1Av alone, or complexed with biotinylated saporin, is not toxic to normal human hematopoietic stem cells due to their lack of TfR1 expression. In addition, the combination of ch128.1Av with non-biotinylated drugs, such as the HOX protein inhibitor peptide HXR9 or the natural chemical compound gambogic acid, results in an additive or synergistic anti-cancer activity against malignant B cells. Importantly, in two disseminated human MM xenograft mouse models, a single low dose of ch128.1Av alone, and even ch128.1, resulted in significant anti-tumor activity. Our central hypothesis is that the overexpressed TfR1 on circulating activated B cells in HIV infection and on malignant AIDS-NHL cells represents a meaningful target for the use of the proposed antibody-based therapeutics, which can be used as a potential treatment for AIDS-NHL, as well as for removing activated B-cells in HIV+ persons in order to prevent the development of AIDS-NHL by ?resetting the B-cell clock?. Importantly, we have already obtained preliminary results demonstrating that human AIDS-NHL cell lines, and activated (but not resting) human B cells, express high levels of TfR1 and that our anti-TfR1 Abs are efficacious against both malignant and activated B cells in vitro and in meaningful mouse models. We have four specific aims: Aim 1: Define the ability of ch128.1Av and ch128.1 to inhibit/eliminate HIV- and/or EBV-activated B cells in vitro; Aim 2: Define the ability of ch128.1Av and ch128.1 to inhibit/eliminate AIDS-NHL in vitro; Aim 3: Define the toxicity of ch128.1Av and ch128.1 on hematopoiesis, and in general, as well as their ability to inhibit HIV- or EBV-driven B-cell activation in vivo; Aim 4: Define the potential of ch128.1Av and ch128.1 in passive immunotherapy for AIDS-NHL. This project will develop the scientific basis for the use of ch128.1Av and ch128.1 alone, or combined with other drugs, for the prevention and treatment of AIDS-NHL.